MathOverflow's primary goal is for users to ask and answer research level math questions, the sorts of questions you come across when you're writing or reading articles or graduate level books. Of course, individual questions don't have to be worthy of an article, and they don't have to be about new mathematics. A typical example is, "Can this hypothesis in that theorem be relaxed in this way?"

From my reading of these FAQs and looking at Cayley_graph wikipedia page, I would expect the only example question so far to be on topic on both sites and given the overlap between the two sites, I think this proposal would have trouble defining questions which aren't on topic on one of these sites, if not both.